"And then they will flow at roughly monthly intervals, so that after we have sent in just two or three of the brigades, I think we will... have a pretty good idea whether at least on the military side the Iraqis have stepped up to the plate in terms of fulfilling their commitments.

But Mr Gates did not say what the US would do if the Iraqi government failed to meet the commitments.

He also said there should be no announcement of a troop pullout, whenever this was to happen.

"I think that any time you announce a specific deadline or specific time-line for departing in a situation as volatile as this, you basically give your adversaries the confidence that all they have to do is wait you out," he said.

But both Democrats and Republicans have continued to criticise the strategy.

Democrat Senator Carl Levin, the committee chairman, said he opposed sending in more troops "without condition, without limitation".

"For America to supply more troops while the Iraqi leaders simply supply more promises is not a recipe for success in Iraq," he said.

Disappointment

The hearing also sought to address concerns about possible military action against Iran.

Gen Pace denied that President Bush's promise to tackle Tehran's interference in Iraq would lead to US military operations inside Iran.

"From a military standpoint, no need to cross the Iranian border," he said.

Mr Bush made his announcement of a "surge" force of troops in a televised speech on Wednesday.

Alongside criticism from Democrats - who now control both houses of Congress - several Republicans have publicly expressed their disappointment with the move, which runs counter to recommendations by the bipartisan Iraq Study Group.

The Democrats have promised a non-binding vote in both houses of Congress on the strategy.